When on VENT for heating, after driving for 10.

When on VENT for heating, after driving for 10 minutes or so, the heat gauge will slowly move to the L on "normal" and then drop back to the O. The rear heat blows ambient temperature.

I have replaced the small single-line vacumm switch that goes to a "t" and then to vacumm reservoir & into heater plenium.

Water pump is new.